Sensor for the detection of combustible gases

ABSTRACT

A sensor for detecting combustible gases in a test gas. The core of this sensor is comprised of a sensitive layer based on a semiconducting metal oxide that is deposited on a ceramic substrate and for which the electrical resistance provides information on the concentration of combustible gases in a test gas. The sensitive layer (3) is comprised of a compound (12) of sintered-together grains (15) of the semiconducting metal oxide, the surface of which is coated with gold and/or a gold alloy. The semiconducting metal oxide in this case is stannic oxide (SnO 2 ), indium oxide (In 2  O 3 ), titanium oxide (TiO 2 ) or another n-semiconducting metal oxide or metal mix oxide. The gold alloy, for example, is composed of 66 mol % gold and 33 mol % palladium (Pd).

PRIOR ART

The invention concerns a sensor for the detection of combustible gasesin a test gas, having a sensitive layer on the basis of a semiconductingmetal oxide that is deposited on an insulating, ceramic substrate, andfor which the electrical resistance provides a statement of theconcentration of combustible gases in a test gas. The invention furtherrelates to a process for the manufacture of a sensitive layer of typefor gas sensors, which determine the combustible gas component in a testgas by evaluating the electrical resistance of the sensitive layer. Theuse of sensors for detecting specific components of a test gas is known,which sensors have a sensitive element composed of a semiconductormaterial, the electrical resistance of which changes when coming incontact with the specific gas component. Sensors of this type are used,in particular, for determining the oxygen content in exhaust gases, e.g.from piston-type internal combustion engines, but also for determiningmethane, carbon-monoxide or alcohol. The semiconductor materials usedare in particular semiconducting metal oxides such as stannic oxide(SnO₂), zinc oxide (ZnO), titanium oxide (TiO₂) or tungsten oxide (WO₃),depending on the purpose. These known sensors normally are produced withthe thick or thin-layer technique. Conductor tracks, by means of whichthe change in resistance is determined later on, are applied onto aninsulating, preferably a ceramic substrate, e.g. made of aluminum oxide(Al₂ O₃), as well as the semiconducting metal oxide. In order toincrease on the one hand the sensor sensitivity--which depends on thetemperature--and to ensure on the other hand that the thermodynamicbalance of absorption and desorption is maintained, it is standardpractice to heat the substrate with the sensor arrangement. Inaccordance with known suggestions, the heating devices needed for thisare arranged, for example, on the underside of the substrate--whereinthe sensor arrangement is located on the top--or they can be integratedinto the substrate or arranged between substrate surface and sensorarrangement. A sensor of this type is known, for example, from the EP-OS313 390. The sensor following from this reference has a substratecomposed of aluminum oxide (Al₂ O₃) with a heating device as well as asensor arrangement placed onto one side. As semiconducting material,stannic oxide (SnO₂) is suggested for detecting methane, tungsten oxide(WO₃) for detecting carbon monoxide or lanthanum nickel oxide (LaNiO₃)for detecting alcohol.

A similar sensor, for which the heating device is integrated into asubstrate composed of aluminum oxide (Al₂ O₃), is known from the DE-OS36 24 217. The gas-sensitive semiconductor layer for this sensor iscomposed of a porous titanium dioxide (TiO₂) enriched with a secondmetal oxide. The described sensor is provided in particular forregulating the air/fuel ratio in an exhaust gas by measuring the oxygencontent.

These known sensors on a semiconductor basis have proven themselves inpractical operations for detecting combustible gases, such as carbonmonoxide (CO), hydrogen (H₂) and hydrocarbons. However, all knownsensors have a tendency to a transverse sensitivity relative to nitricoxides, such as occur, for example, in the gaseous atmosphere duringcombustion processes or in automotive exhaust gases. If used innitric-oxide containing test gases, the known sensors therefore provideinaccurate results or require special correction devices.

It is the object of the invention to provide a sensor as well as aprocess for manufacturing a sensitive layer, which permits asufficiently good detection of the share of combustible gases even intest gases with a nitric-oxide component.

The above object is generally achieved according to a first aspect ofthe invention by a sensor for detecting combustible gases in a test gas,including a sensitive layer on the basis of a semiconducting metal oxidethat is deposited on an insulating, ceramic substrate, and for which theelectrical resistance delivers a statement on the concentration ofcombustible gases in a test gas, wherein the sensitive layer has astructural composition of sintered together grains of the semiconductingmetal oxide and the surface of this composition is coated with goldand/or a gold alloy.

The above object is generally achieved according to a further aspect ofthe invention by a process for manufacturing a sensitive layer for gassensors, which determine the combustible gas component in a test gas byevaluating the electrical resistance of the sensitive layer, includingthe following steps:

producing a powder on the basis of a semiconducting metal oxide withcomponents of an oxide that increases the conductivity, of palladium, aswell as of a 2- or 3-valent element of the alkaline earth or the rareearths;

coating the powder grains with gold and/or an alloy of gold and one orseveral precious metals of the group palladium (Pd), platinum (Pt),rhodium (Rh), iridium (Ir), osmium (Os);

producing a paste from the coated powder grains;

applying the paste to a sensor substrate as the sensitive layer; and

sintering the sensitive layer on the substrate at 500° to 1000° C.,preferably 700° C.

The inventive features of the sensor defined by independent claims 1 to10 can be produced with techniques that are known on principle and thuscan be produced cost-effectively. It reacts swiftly and with goodsensitivity to combustible gases contained in a test gas, without beingsimultaneously affected by nitric-oxide components in the test gas. Itis therefore particularly well suited for use in motor vehicles, e.g. tocontrol the ventilation of the inside area, which it may interrupt ifexhaust gases reach the inside as well as for measuring smallconcentrations of combustible gases in the air or in exhaust gases fromfirings of combustion engines.

Advantageous modifications and useful embodiments of the inventivesensor respectively the process for manufacturing it follow from thedependent claims.

The sensitivity and speed of the sensor with respect to thepredetermined use can be adjusted by doping the semiconducting oxideused for the sensitive layer with palladium (Pd).

The excellent function of the suggested sensor is due to the inventivecoating of the sensitive semiconductor oxide with gold or a gold alloy.It prevents the absorption of the nitric-oxides occurring in the testgas. As a result of this, the sensor can detect low concentrations ofcombustible gases, such as are standard for road traffic, with a stablebasic resistance, without experiencing a permanent increase in theresistance or a drift if nitric-oxide containing gases impinge on thesensor.

The suggested sensor furthermore is well suited for a summarydetermination of the content of combustible gases in the exhaust gasfrom furnaces in dependence on its oxygen content.

It is advantageous if an oxide that increases conductivity, e.g.,tantalum oxide, is added to the metal oxide, which is used as basematerial for the sensitive layer. Another, advantageous improvement ofthe sensor effect is achieved by doping the base material with amaterial that acts as a diffusion blocking agent, especially palladium.It is also advantageous if a bivalent or trivalent element of thealkaline earths or the rare earths is added to the basic material forthe sensitive layer, for example, magnesium oxide, to prevent the metaloxide that is present in the form of grains from sintering together toostrongly.

It is preferable if the semiconducting metal oxide used is stannicoxide.

The suggested sensor as well as the process for manufacturing asensitive layer are explained in the following in more detail and withthe aid of the embodiments shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show stages in the production of a sensor as seen fromabove. FIG. 4 shows a view from the side of the sensor. FIG. 5 shows thesensor in a view from below. FIG. 6 is an enlarged section of thesensitive semiconductor layer prior to the sintering. FIG. 7 shows asection of the sensitive semiconductor layer after the sintering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a view from above of a sensor with a rod-shaped design. Thebasis for this sensor is a substrate 1, composed of an electricallyinsulating and heat-resistant material, preferably aluminum oxide (Al₂O₃), onto which the additional components that form the sensor aredeposited in the thick/or thin layer technique. Two conductor tracks 2with comb-type interlacing of their head ends are initially deposited ona surface of substrate 1, which is referred to in the following as theupper side.

In the region of the interlaced ends, a sensitive, semiconducting metaloxide layer 3 is deposited according to FIG. 2 over the conductor tracks2 with a layer thickness of 5 to 500 μm, preferably 20 to 50 μm. Themetal oxide suitably is a stannic oxide (SnO₂). However, alternativelyit is also possible to use indium oxide (In₂ O₃), titanium oxide (TiO₂)or another n-semiconducting metal oxide or metal mix oxide, to which aconductivity-enhancing doping element has been added in concentrationsof 0.001 to 0.5 mol %, preferably 0.005 to 0.015 mol %. If stannic oxide(SnO₂), tantalum (Ta₂ O₃), niobium oxide (Nb₂ O₃) or mixtures thereofare used, then antimony or tungsten oxides are used as doping elements,while stannic, titanium or Cer-oxides are added if indium oxide (In₂ O₃)is used. The metal oxide layer 3 is furthermore doped homogeneously witha precious metal additive. It consists preferably of palladium (Pd) in aconcentration of 0.5 to 3 mol %, in particular 1.2 mol %, which limitsthe subsequently deposited gold coating from diffusing into the metaloxide. The precious metal admixture can additionally contain percentagesof platinum (Pt) and/or rhodium (Rh) in a concentration of 0.001 to 0.3mol %, which influence the response speed of the sensor. The metal oxideof layer 3 can also contain admixtures for limiting the crystallitegrowth following the conclusion of the production process, in particularto prevent the further sintering together of the semiconductor oxidethat initially is present in the form of grains, and can thus improvethe resistance to aging of the sensors. Suitable admixtures are theoxides of bivalent elements, e.g. magnesium (Mg), barium (Ba), calcium(Ca), strontium (Sr), zinc (Zn) or a trivalent element such as aluminumas oxide (Al₂ O₃) in concentrations of 0.01 to 0.3 mol %.

A further component of the metal oxide layer 3 is gold (Au) in aconcentration of 0.3 to 3 mol %, preferably 0.6 mol %, or an alloy ofgold and one or several precious metals from the group palladium (Pd),platinum (Pt), rhodium (Rh), iridium (IR), osmium (OS) or silver. Asshown in FIG. 7, the gold or gold alloy is not mixed homogeneously withthe other components of the metal oxide layer 3, but is deposited suchthat it forms a surface coating 14 for the structural compounds orcomposite 12, which are composed of the doped metal oxide. The latterdevelop during the production by sintering together a basic materialthat initially is present in the form of grains 15, as shown in FIG. 6.The sponge-like structure of the sensitive layer 3 as shown in FIG. 7,which is composed of the metal oxide with a surface coating of a goldalloy, is responsible for the desired transverse sensitivity of thesensor relative to nitric-oxides. It forms an essential characteristicof the inventive sensor.

As can be seen in FIGS. 3 to 5, a porous protective layer 4 can bedeposited over the metal oxide layer 3, advisably with a thickness ofapproximately 10 to 100 μm. This layer preferably is composed ofaluminum oxide (Al₂ O₃), titanium oxide (TiO₂), zirconium oxide (ZrO₂)or silicon dioxide (SiO₂). A heating arrangement 5 is located on theopposite side of substrate 1, meaning on the underside, which is shownin FIG. 5 in a view from above. It consists of a meandering conductortrack in a region below the sensitive region of the sensor.

A process for manufacturing a metal oxide layer with a sponge-likestructure corresponding to FIG. 7 is described in the following. Allquantities given are applied to a composition of one mol stannic oxide.

In an introductory processing step, one mol stannic tetrachloride(SnCl₄), together with 0.02 mol % tantalum V-chloride (TaCl₅) are addedto 500 ml hydrochloric acid (HCl). The mixture is dissolved in 30 lwater. Stannic oxide (SnO₂) is then precipitated out from this waterymixture by adding ammonia (NH₃). The precipitated out product issubsequently washed several times, advisably at least three times,through sedimentation. The resulting sediment is then dissolved in 300ml water by adding 1.2 mol % palladium nitrate and 0.1 mol % magnesiumnitrate and is subsequently dried. The product that exists following thedrying is calcined for five hours at a temperature of preferably 500° C.and is then ground. It is useful if the resulting grain size is in arange of 1 μm.

The grinding delivers a powder composed of stannic oxide (SnO₂) withshares of 0.01 mol % tantalum oxide (Ta₂ O₅), 1.2 mol % palladium (Pd),as well as 0.1 mol % magnesium oxide (MgO). The tantalum oxide (Ta₂ O₅)in this case serves to increase the conductivity, the palladium (Pd)functions as diffusion blocking agent for the subsequent surface coatingof the powder grains, while the magnesium oxide (MgO) functions tocontrol the degree of sintering, in particular it prevents a strongsintering together of the grains during the production process. Thepowder grains have a crystallite microstructure with a crystallite sizeof approximately 20 to 80 nm.

Following that, a coating is deposited over the powder. In a first step,0.4 mol % gold (Au) in the form of gold acid (HAuCL₄) dissolved in wateris deposited over the powder, resulting in a slurry. This slurry isdried. All the gold acid existing in the slurry is subsequently reducedto gold through thermal treatment in a rotary furnace in a water jetwith 10% H₂ in N₂, preferably at approximately 200° C. In a second step,a palladium coating follows this coating with gold. For this, 0.2 mol %palladium (Pd) in the form of palladium nitrate dissolved in water isdeposited on the powder previously coated with gold. Again a slurrydevelops, which is then dried. The dried slurry is treated thermally ata temperature of preferably approximately 200° C. in a rotary furnaceand in an air flow. The palladium (Pd) present in the slurry is therebyconverted to palladium oxide (PdO) and palladium-coated grains develop.

It is useful if a screen printing paste is produced from the coatedpowder with the aid of butyl carbitol and ethylcellulose. For theproduction of a sensor, this paste is applied as sensitive layer 3 overthe substrate 1. FIG. 6 illustrates the structure of such a sensitivelayer 3 that is applied to a substrate 1. It is composed of adjoininggrains 15, the surfaces of which are enclosed in a layer 11. The grains15 themselves are composed of individual crystallites 10.

The sensors imprinted with the sensitive layer 3 are advisably sinteredat 700° C. for about three hours. During this process, the palladiumoxide (PdO) present in the layers 11 that envelope the grains 15 changesto palladium (Pd) and alloys with the gold that also exists in theenveloping layers 11. The stannic oxide grains 15 that are initiallyseparate combine to form structural compounds 12 during the sintering,and the enveloping layers 11 that initially surround the individualgrains 15 grow together to form a metal coating 14 in the form of smallclusters covering the surfaces of the structural compounds 12. FIG. 7shows the structure of a sensitive layer 3 following the sintering.

It was found that for an operating temperature of approximately 300° C.,sensors with a sensitive layer composed of a material produced accordingto the inventive process when admitted with 40 vpm carbon monoxide (CO)have in air with a relative humidity of 60% as referred to 20° anelectrical resistance reduced by a factor of 3 to 5 relative to thevalue in pure air. When admitting a test gas additionally with 1 to 5vpm nitric-oxide (NO₂), this results in an increase in the electricalresistance of less than 30% for 40 vpm carbon monoxide (CO) and 1-5 vpmnitric-oxide (NO₂).

We claim:
 1. Sensor for detecting combustible gases in a test gas whichsensor displays accuracy and sensitivity even in the presence of nitricoxides, with a sensitive layer on the basis of a semiconducting metaloxide that is deposited on an insulating, ceramic substrate, and forwhich the electrical resistance delivers a statement on theconcentration of combustible gases in a test gas and wherein thesensitive layer (3) has a structural composition (12) of sinteredtogether grains (15) of the semiconducting metal oxide and the surfaceof sintered together grains in this composition (12) is coated with goldand/or a gold alloy to prevent adsortion of any nitric oxides onto saidsurface from said test gas.
 2. Sensor according to claim 1, wherein thegold alloy is composed of 66 mol % gold and 33 mol % palladium (Pd). 3.Sensor according to claim 1, wherein the gold alloy is composed of goldand one or several precious metals from the group palladium (Pd),platinum (Pt), rhodium (Rh), iridium (IR), osmium (Os) or silver (Ag).4. Sensor according to claim 1, wherein the alloy is composed of goldwith a component of another metal with higher melting point.
 5. Sensoraccording to claim 1, wherein the component of the gold and/or the goldalloy relative to the semiconducting metal oxide is 0.3 to 3 mol %. 6.Sensor according to claim 1, wherein the semiconducting metal oxide hasa homogeneous doping with palladium of 0 up to 3 mol %.
 7. Sensoraccording to claim 1, wherein the semiconducting metal oxide is stannicoxide (SnO₂).
 8. Sensor according to claim 1, wherein the semiconductingmetal oxide is indium oxide (In₂ O₃), titanium oxide (TiO₂) or anothern-semiconducting metal oxide or metal mix oxide.
 9. Sensor according toclaim 1, wherein the semiconducting metal oxide is doped with tantalumoxide (Ta₂ O₅) or niobium oxide (Nb₂ O₅) at a concentration of 0.001 to0.05 mol % relative to the semiconducting metal oxide.
 10. Sensoraccording to claim 1, wherein the semiconducting metal oxide is mixedwith a 2-valent or 3-valent element of the alkaline earth or the rareearths, at a concentration of 0.03 to 0.3 mol % and preferably 0.1 mol%.
 11. Sensor according to claim 1, wherein the sensitive layer has asponge-like structure.
 12. Sensor according to claim 5, wherein thecomponent of the gold and/or the gold alloy relative to thesemiconducting metal oxide is approximately 0.6 mol %.
 13. Sensoraccording to claim 6, wherein the semiconducting metal oxide has ahomogeneous doping with palladium of 1.2 mol %.
 14. Sensor according toclaim 9, wherein the semiconducting metal oxide is doped with tantalumoxide (Ta₂ O₅) or niobium oxide (Nb₂ O₅) at a concentration of 0.005 to0.015 mol % relative to the semiconducting metal oxide.
 15. Process forproducing a sensitive layer for gas sensors, which determine thecombustible gas component in a test gas by evaluating the electricalresistance of the sensitive layer, comprising the followingsteps:producing a powder (15) on the basis of a semiconducting metaloxide with components of an oxide that increases the conductivity, ofpalladium as well as of a 2- or 3-valent element of the alkaline earthor the rare earths; coating the powder grains (15) with gold or an alloyof gold and one or several precious metals of the group palladium (Pd),platinum (Pt), rhodium (Rh), iridium (Ir), osmium (Os); producing apaste from the coated powder grains; applying the paste to the sensorsubstrate (1) as the sensitive layer (3); and sintering the sensitivelayer (3) on the substrate (1) at 500° to 1000° C.
 16. Process accordingto claim 15, wherein the production of the powder involves the followingsteps:mixing stannic tetrachloride (SnCl₄) and tantalum pentachloride(TaCl₅) into hydrochloric acid (HCl), dissolving the mixture in water,precipitating out stannic oxide (SnO₂) from the solution, sedimentationof precipitated product, dissolving sediments in water, drying solution,calcining dried product, and grinding the calcined product.
 17. Processaccording to claim 15, wherein the powder grains are coated initiallywith gold and subsequently with palladium.
 18. Process according toclaim 15, wherein the coating of the powder grains (15) with gold (Au)involves the following steps:producing a slurry by adding auric aciddissolved in water to the powder, drying the slurry, and thermallytreating the slurry in a water flow.
 19. Process according to claim 15,wherein the coating of the powder grains (15) with palladium (Pd)involves the following steps:producing a slurry by adding palladiumnitrate (Pd(NO₃)₂) dissolved in water to the gold-coated powder (15);drying the slurry; and thermally treating the slurry in the air flow.20. Process according to claim 15, wherein the semiconducting metaloxide is stannic oxide (SnO₂).
 21. Process according to claim 15,wherein the oxide that increases conductivity is tantalum oxide (Ta₂O₅).
 22. Process according to claim 15, wherein magnesium oxide (MgO) isused as alkaline earth element.
 23. Process according to claim 15,wherein the sintering of the sensitive layer takes place atapproximately 700° C.